The invention relates to a TV-RF input circuit provided with an RF coupling device via which an aerial input is coupled to first, second and third parallel tunable RF resonant circuits for a parallel selection of a desired frequency in first, second and third TV frequency bands substantially succeeding one another in frequency, said RF coupling device having a high-pass .pi.-section for suppressing frequencies below the first TV frequency band and being provided with a capacitive series branch and first and second inductive shunt branches, said .pi.-section being coupled via a first series inductance to the first RF resonant circuit.
A TV-RF input circuit of this type is used in the TV tuner of the type UV 636 of Philips, described in the "Philips Data Handbook" Book C2, entitled "Television tuners", published by Philips Electronic Components and Materials of Eindhoven, February 1987.
In the known TV tuner the first, second and third RF resonant circuits are arranged in first, second and third signal paths, respectively, which are mutually parallel arranged between the coupling device, on the one hand, and a TV-IF output of the tuner on the other hand. The entire TV reception signal of the aerial is applied in a broad band via the RF coupling device to each of the RF resonant circuits. The RF resonant circuits are parallel tunable from one common tuning voltage to TV frequency bands succeeding one another substantially contiguously. With the aid of the first, second and third RF resonant circuits, a TV channel is selected in TV frequency bands of approximately 50 MHz to 270 MHz, 280 MHz to 400 MHz and 400 MHz to 800 MHz, respectively. Each signal path comprises an amplifier stage arranged downstream of the RF resonant circuit, which stage also functions as a band switch, and is followed by a further tunable TV channel filter and a mixer stage to which an oscillator mixing signal is applied from a tuning oscillator. One of the signal paths is activated by means of a switching signal, that is to say, the TV channel selected in the RF resonant circuit preceding the band switch in one of the signal paths is switched through via this band switch for a further selection and conversion into a TV-IF signal.
However, the impedance of the RF resonant circuits varies considerably with the tuning frequency. The RF coupling device should nevertheless maintain a correct impedance matching between the aerial input, on the one hand, and the RF resonant circuits, on the other hand, within the entire TV reception frequency range and it should also prevent the RF resonant circuits from noticeably loading one another.
In the RF coupling device of the known TV-RF input circuit, this is achieved by means of a switching device with which the input of the first RF resonant circuit is connected to ground when activating the third signal path, and by means of inductive coupling elements between the aerial input, on the one hand, and each of the RF resonant circuits, on the other hand. The high pass .pi.-section is arranged in the known RF coupling device ahead of the first RF resonant circuit and suppresses signals below the TV frequency range which may give rise to interference, for example, radio signals and signals at the TV image and sound intermediate frequency.